I used to reference the RDA measures for water intake and discuss 3.2L for men and 2.9L for women to my clients. I would mention that some hydration comes from your food (like the 9-13 servings of fruits and vegetables you should be eating everyday) and some from the liquids you drink (straight water and other beverages). Empathetically, I might quote a company that manufactures high tech body composition testing equipment who use total body water readings and admit to using a suboptimal average in their calculations because virtually everyone is dehydrated to a degree.